It is well known that high-pressure discharge lamps, when hot, require a substantially higher ignition or firing voltage than when they are cold. If a high-pressure discharge lamp is deenergized, and should be re-started or ignited while still hot, the ignition voltage required for hot-starting is substantially higher than when the lamp has been permitted to be cooled. Many applications require high-pressure discharge lamps which are capable of being hot-started or re-started, as required, that is, can be turned ON, then OFF, and again turned ON, without awaiting an intermediate cooling phase.
Bases for such lamps are subject to extreme requirements with respect to resistance against arc-over and acceptability of high voltages. The bases must fit standard sockets.
High-pressure discharge lamps to which the present invention relates are generally described in the referenced U.S. Pat. No. 4,542,316, Hall et al, as well as in the British Patent 2 100 404, Ainsworth et al. For example, the British patent describes a lamp which has improved resistance to high voltages and can be re-started when hot. This lamp, however, is of limited power, and lamps of that type have a power rating below that to which the present invention relates. The British patent describes use of a ceramic separating strip or plate which extends between the contact terminals or contact pins extending from the lamp base, as well as between connecting leads to the contact pins or terminals. This strip may be part of the socket engaging in the slot of the lamp base between the contact pins or posts, or may be formed unitary with the ceramic base and fitting into a matching slit, slot, or depression in the lamp socket.
U.S. Pat. No. 4,542,316, Hall et al, is directed to a single-ended high-pressure discharge lamp which, also, can be re-started while hot. The lamp base has an improved high-voltage insulation formed by an axially extending strip of mica between the connecting leads of the lamp, located in a slot cut into or formed in the pinch seal of the lamp bulb. The mica strip is intended to suppress corona and creep currents between the current supply leads.
The lamps described have a base in which the high-voltage insulation is sufficient when the lamp itself has low or intermediate electric power rating. As the power rating for lamps increases, experiments have shown that the high-voltage resistance of the known bases is still not high enough in order to permit hot re-ignition of high-power single-ended high-pressure discharge lamps. High-pressure discharge lamps, thus, having power ratings of, for example, about 6,000 to 12,000 watts, require pulse voltages for re-ignition in the order of about 70,000 volts.